1. Field of the Invention
This invention relates to a camshaft for internal combustion engines. More particularly this invention relates to a camshaft having a lightweight and excellent vibration-damping properties, and having a structure such that the camshaft is easily fabricated, and thus is suitable for a mass production thereof.
2. Description of the Related Art
In the manufacture of automobiles a serious problem is a reduction of the weights of the various parts and a suppression of noise produced when driving.
In this regard, Japanese Unexamined Utility Model Publication (Kokai) No. 61-94210 discloses a camshaft having a shaft portion formed as a pipe formed by laminating two steel plate with a viscoelastic high molecular weight resin therebetween i.e. the camshaft is made of a composite vibration-damping steel plate.
Nevertheless, the camshaft disclosed in Japanese Unexamined Utility Model Publication (Kokai) No. 61-94210 is formed by bending a composite vibration-damping steel plate to form a pipe having a seam closed by a welding process, and as a result, has following defects.
The high temperature necessary for the welding decomposes the high molecular weight resin at the seam portion and accumulates decomposed gas, and thus the expected vibration-damping properties are not exhibited.
As a measure for solving the above problem, there may be thought of a method of injecting a reactive liquid, such as polyamide oligomer (monomer), into the space between the outer pipe and inner pipe forming the double pipe structure, and carrying out a high degree polymerization, or making three-dimensional polymer by using a catalyst or heat.
Such a method, however, needs long reaction time, and as it is difficult to make the reaction rates uniform, this method is not suitable for mass production. Moreover the method has a problem of a leakage of unreacted products, which corrode the metal portions
Furthermore, the anion polymerization used for the above reaction requires a moisture control, etc., and thus many problems arise in an industrial production of such a shaft.